If there is enough stud sticking out, grab it with some reverse tooth pliers and hope to god it doesn't snap again. If there isnt enough threads left to grab onto then try and weld a nut in the end if there is enough room.
If all else fails you'll have to either drill it right out, or drill it out and use a screw extractor (easyout) to get it out.
Before you try anything I would soak the ones you need to get out in PB blaster overnight
Goodluck.